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1.7 Steel bomb calorimetry

Calibration

Calibration of the bomb calorimeter is performed by detonating a known quantity of AR grade benzoic acid under 25 atmospheres pressure oxygen with the bomb being kept inside of a thermostated jacket of circulating water. The circulated water jacket is to keep the temperature of the water surrounding the bomb at a constant temperature.

The mass of benzoic acid needs to be known to 4 decimal places. Also, the mass of the cotton used as the fuse must be known as this too will have a calorific value.

The literature value for delta Hc benzoic acid is 26.435 kJ g-1 (3228.3268 kJ mol-1) with an energy equivalent, W, for the calorimeter of 10.7609 kJ K-1.

The temperature readings in the tables below are from a Beckman thermometer. This type of thermometer normally ranges from between 0 to 6C, is highly accurate and can be adjusted to make reading a temperature simpler. The temperatures are all therefore relative to the initial temperature taken by the Beckman thermometer.

Time (secs) Temp (C) Time (secs) Temp (C)
0.00 1.005 23.00 3.560
3.00 1.020 23.15 3.620
6.00 1.040 23.30 3.660
9.00 1.060 23.40 3.700
12.00 1.080 23.50 3.730
15.00 1.100 24.00 3.770
18.00 1.130 24.15 3.785
21.00 1.150 24.30 3.800
21.10 1.160 24.40 3.810
21.20 1.250 24.50 3.820
21.30 1.580 25.00 3.830
21.40 2.080 26.00 3.850
21.50 2.380 27.00 3.855
22.00 2.630 28.00 3.855
22.10 2.990 29.00 3.855
22.20 3.160 30.00 3.850
22.30 3.280 31.00 3.845
22.40 3.400 32.00 3.840
22.50 3.490 33.00 3.835
34.00 3.835
35.00 3.830

Table 1. Calibration data for benzoic acid, mass 1.0870g

When plotted, this gives the following graph

Calculations

T1 = 1.005 t1 = 0

T2 = 1.15 t2 = 21

T3 = 3.855 t3 = 26.5

T4 = 3.81 t4 = 44

qi and qn are the thermal contribution of the cotton and fuse wire respectively.

As these values are for approximately 1g of benzoic acid, it is far simpler to compare the results from the experiment to the calibration results, gram to gram. This saves a great deal more time than for the constant re-calculation of each sample.

The graphs

It can be seen from both of these graphs that the amount of energy released per gram by both the dry agar and the dry NI3 agar powder are rather high and considerably more than for the benzoic acid, though this is not true for the agar powder itself.

Calculation of energy release

It has already been show how to calculate the both the energy equivalent and from (1) the total energy is derived.
Agar powder Agar / NI3
t1 0 s T1 0.02 t1 0 s T1 0.54
t2 2 s T2 0.08 t2 3 s T2 0.71
t3 5 s T3 3.55 t3 12 s T3 5.385
t4 9s T4 3.56 t4 14 s T4 5.84
Tr 0.49 Tr 2.64

(T = C, t = seconds. Tr calculated by the Simpson rule (see equation (14))

Calculating these out with the formulae above yield the following results
Sample Agar NI3
T1 0.020 0.540 Mass (agar) 0.800 g
T2 0.080 0.710 delta T (agar) 3.460
T3 3.550 5.835
T4 3.560 5.840 delta E / kJ 37.238
t1 0.000 0.000
t2 2.000 3.000
t3 5.000 12.000 Mass (NI3) 1.130 g
t4 9.000 14.000 delta T (NI3) 5.036
Ra 0.003 0.002
Rf 0.030 0.057 delta E / kJ 54.190
Ta 3.555 5.838
Tf 0.050 0.625
Tr 3.47 5.125

It is obvious from the results that both NI3 - agar and agar alone give off far more energy per gram than AR benzoic acid.

However, it should also be remembered that the NI3 is in agar. The actual mass of agar was the same as for the agar / NI3 sample itself, therefore if the difference is taken between the two delta E values (as well as between the masses), the final figure for NI3 will be

This energy term can be related to delta H as all energy changes within the bomb are delta E values . By using

where n is the number of moles of gas evolved and T is the average run temperature . The nRT term will be negligible here.

If the delta E value is divided by 0.33 (the mass of NI3) then multiplied by the R.M.M. of dry NI3 (392.7), delta Hdet is found.

This value is far higher than the value of delta Hf NI39 (287 23 kJ mol-1).


9  Davies, R.H., Finch, A., Gates, P.N. The Standard Enthalpy of Formation of Nitrogen Tri-Iodide Monoammine and the Nitrogen Iodine Bond Energy.J. Chem. Soc - Chem. Comms, 1989, No. 19, pp 1461 - 1462.


For an energy release of such an order above that of the published value for delta Hf, the nature of the type of bond in NI3 comes into question. While there is no doubt that there is a bond of some description, the sheer size of the iodine atoms in comparison to that of the central nitrogen atom would suggest large bond lengths and a high degree of sp or pd hybridisation. The bond would also have a substantial degree of anti-bonding character. The bond energy would be expected to be low.

In dealing with a molecule of this type, consideration of bond energies is not enough. As each nitrogen - iodine bond is broken, the remained iodine atoms are drawn closed to the nitrogen ion. This results in a far shorter bond length which requires more energy to break the remaining bonds. The energy required to break the second and third N - I bonds would have to be higher with the final bond energy a more typical figure.

Glass calorimetry. Method 1.8

1. Speed of heat transfer from the glass calorimeter to water.

The rate at which the glass transmitted heat to the water was required to ensure a rapid enough response. The thermal capacity of the polystyrene cup was considered to be negligible. If the heat transfer rate is not fast enough, the experimental data recorded will be not accurate enough.

The computer program was started. The bomb was filled with water at 25C, placed into the cup and water at 20C poured around it. The lid was quickly placed on and the time taken for the temperature to rise by 1C recorded. This was repeated three times.
Run number Time taken for 1C rise
1 6 seconds
2 5.8 seconds
3 6.3 seconds

The transmission of heat to the water was considered to be rapid enough for the calorimetry. A slow time would indicate that the heat transfer across the glass would not be rapid enough and substantial cooling may occur within the bomb. The water would also warm far slower giving a false depiction of the calorimetric experiment in progress. A rapid heat transfer would indicate the reverse.

2. Calibration and work equivalent

Using the same method as in 1.7 (except for the extreme O2 pressure), the glass calorimeter was calibrated using 0.2g benzoic acid.

The following results were obtained.

Graph of calibration curve for benzoic acid using the glass calorimeter.

Time (secs) Temp (C) Time (secs) Temp (C)
0.00 0.135 23.00 0.479
3.00 0.137 23.10 0.487
6.00 0.140 23.25 0.492
9.00 0.143 23.38 0.498
12.00 0.145 23.50 0.502
15.00 0.148 24.00 0.507
18.00 0.152 24.10 0.509
21.00 0.155 24.30 0.511
21.10 0.156 24.40 0.512
21.20 0.168 24.50 0.514
21.30 0.213 25.00 0.515
21.40 0.280 26.00 0.518
21.50 0.320 27.00 0.518
22.00 0.354 28.00 0.518
22.10 0.402 29.00 0.518
22.20 0.425 30.00 0.518
22.30 0.441 31.00 0.517
22.40 0.457 32.00 0.516
22.50 0.469 33.00 0.516
34.00 0.516
35.00 0.515

The temperature rise was calculated via the computer program by taking the new temperature reading of the water from the initial temperature reading of the water.

From the calibration data
T1 0.073
T2 0.095
T3 0.785
T4 0.785 Mass (g) 0.20
t1 0.000 delta T 0.688
t2 0.403
t3 1.614
t4 1.883
Ra 0.000
Rf 0.008
Ta 0.785
Tf 0.084
Tr 0.689

The work equivalent, W, was calculated on the basis of this data and gave a result of 7.7955 kJ K-1.

3. Calorimetry

The computer was reset and the program restarted. A known mass of NI3 was placed into the glass calorimeter, the calorimeter sealed and placed into the polystyrene cup, which was sealed also. The spark was introduced and temperature rise observed. The following results were observed for three different samples.

The delta Hdet (NI3) was calculated using the same method as for the steel calorimeter results.

Sample 1 2 3
T1 0.25 0.31 0.18
T2 0.43 0.50 0.35
T3 2.53 2.57 2.11
T4 2.54 2.58 2.12
t1 1.000 1.000 1.000
t2 2.700 2.600 2.650
t3 6.000 6.150 5.800
t4 6.100 6.500 6.000
Ra 0.100 0.029 0.050
Rf 0.106 0.119 0.103
Ta 2.535 2.575 2.115
Tf 0.340 0.405 0.265
Tr 2.100 2.070 1.760
Mass (g) 0.30 0.32 0.27
delta T 1.77 1.89 1.57
delta E / kJ 13.77 14.77 12.24
delta Hdet kJ mol-1 18022.51 18120.63 17806.82
 

delta Hdet is for the dry NI3 (R.M.M. 393). The accuracy of this method gave a higher degree of reproducibility.